Approximately one million people per year have cardiac arrests in the United States. Less than 10 percent of these people are discharged from the hospital live. This percentage of people discharged would increase if the treatment available after the onset of cardiac arrest was improved. Four areas in which the treatment could be improved include artificial circulation during cardiopulmonary resuscitation (CPR), defibrillation countershock techniques, cardiac pacing and cardiac monitoring.
The heart of a human being lies between the sternum and the spinal column. The esophagus is normally a flaccid collapsed tube that lies in the midline of the body between the heart and the spinal column. The anterior surface of the esophagus contacts the posterior surface of the heart and particularly the left ventricle of the heart. The descending aorta is adjacent to the esophagus and the heart and lies between the heart and the spinal column.
The blood of a cardiac arrest patient is artificially circulated during CPR by cyclically compressing and releasing the chest. One theory that describes how artificial circulation is generated during CPR says that the patient's rigid sternum is pushed against the anterior of the heart during chest compression and the heart is squeezed between the sternum and the spinal column. The soft, flexible esophagus is not rigid enough to provide rear support for the heart. Therefore, the sternum must be pushed far enough to force the posterior of the heart against the rigid spinal column in order to produce blood circulation. For CPR to be effective, the blood flow produced must provide perfusion to the heart muscle, known as the myocardium, and the brain, known as the cerebrum, in order for these tissues to remain viable. For the most part, the method of CPR used today produces inadequate myocardial and cerebral perfusion.
In addition to artificial circulation, many patients also require a defibrillation countershock during CPR in order to restart their heart. Defibrillation countershock therapy involves placing two electrodes near the heart and inducing a flow of electrical current through the chest and heart and preferably through the left ventricle of the heart which is the largest part of the heart muscle that is fibrillating. The electrodes used are hand-held paddles or adhesive pads, either of which are placed at different positions on the external surface of the patient's chest, sides and/or back. A defibrillation countershock delivered with this electrode placement methodology is commonly called an external defibrillation countershock.
A sufficient electrical current density must be induced in the myocardium in order to defibrillate a fibrillating heart. Current density is defined as the amount of current per cross sectional area of tissue. In addition, the required minimum current density must depolarize at least a certain minimum critical mass of the left ventricle of the heart in order to achieve defibrillation. For any given total current induced in the chest during a defibrillation countershock, the current density in the myocardium is generally inversely proportional to the distance between the countershock electrodes. This distance will vary depending on the location of the electrodes and the size of the patient's chest. If the electrodes are widely separated, more of the total current will pass through non myocardial tissue. It is therefore advantageous to position the electrodes as close to the heart as possible in order to achieve defibrillation.
The machine used to deliver a defibrillation countershock as well as monitor and, when necessary, pace a patient's heart, is commonly called a defibrillator. All defibrillators used clinically today are described as energy defibrillators in that the person administering countershock therapy presets the amount of electrical energy to be delivered to the patient in the countershock. For any preset energy level, the total current and current density induced in the myocardium is generally inversely proportional to the electrical impedance of the tissues lying between the electrodes. Although the myocardium has a relatively low impedance to current flow, tissues such as bone have a high impedance. For instance, structures such as the sternum, ribs and spinal column have relatively high impedance to current flow. Some or all of these tissues interpose the electrodes during an external countershock. It is therefore advantageous to position the electrodes so that there is the least possible amount of non myocardial, high impedance tissue between them.
It is further advantageous to use the smallest amount of current necessary to defibrillate the heart of a patient in cardiac arrest. Excessive current and specifically excessive myocardial current density causes irreversible structural damage to the myocardial tissue.
Internal countershock therapy utilizes the most ideal electrode placement and offers the highest probability of achieving defibrillation. In this method, the pair of electrodes are placed on opposite sides of and touching the left ventricle of the exposed heart and the current is induced between the two electrodes. Under this circumstance, the distance between the electrodes is minimized and virtually no other tissues other than the myocardium interposes the electrodes. Virtually all of the current flows through the left ventricle of the heart. This electrode placement requires that the chest be opened in order to expose the heart. Therefore, it is typically only performed under sterile conditions in an operating room. This procedure is impractical in an emergency setting outside the operating room.
One newly proposed method of electrode placement meant to reduce the amount of high impedance tissue between the electrodes as well as reduce the distance between the electrodes involves placing one small electrode in the esophagus and a second electrode on the outer surface of the patient's chest. This is shown in U.S. Pat. No. 5,170,803. Similarly, U.S. Pat. No. 4,198,953 shows an esophageal electrode urged against the esophageal wall by a balloon. That patent refers to U.S. Pat. No. 4,090,518, described as suitable for use with the invention of that patent.
A second proposed orientation of electrodes consists of a pair of small electrodes at different spaced positions in the patient's esophagus. This is shown in U.S. Pat. No. 4,960,133. U.S. Pat. No. 4,706,688 also shows placement of multiple electrodes in the esophagus and uses a flaccid balloon to urge the electrodes toward the heart.
A third proposed orientation of electrodes consists of a small electrode placed in the stomach, forced upwardly against the top of the stomach wall until the stomach wall contacts the bottom of the heart. In this device, the second electrode needed for the countershock is placed on the patient's chest. This is shown in U.S. Pat. No. 5,197,491. None of these newly proposed electrode orientations utilize the stomach and the esophagus concurrently for countershock therapy. None of the devices which are placed in the esophagus function as rigid supports to enhance artificial circulation during CPR. Each of these proposed electrode orientations utilize an external electrode. Under this circumstance, the distance between the countershock electrodes may be reduced but is not minimized and high impedance tissues such as the sternum and ribs still interpose the electrodes.
A fourth proposed electrode orientation consists of six electrodes at different locations in the esophagus. Countershock current is passed from the proximal three electrodes to the distal three electrodes. Although the impedance is low with this electrode orientation, the heart does not interpose the electrodes but rather lies anterior to the electrodes and, therefore, a sizable portion of the current induced between the electrodes may pass through non myocardial tissue. This device also does not provide rigid support for the heart during CPR.
U.S. Pat. Nos. 5,056,532 and 5,179,952, while not directed to countershock, do show electrodes placed in the esophagus for pacing and monitoring. U.S. Pat. No. 5,056,532 also shows flaccid esophageal and stomach balloons for positioning the device but having no electrical function.
There are several inflatable devices described in the prior art that are placed in the esophagus during artificial ventilation and act as airway adjuncts by closing off the esophagus to allow air to flow primarily into the lungs. There are other devices which are placed in the esophagus to stop bleeding from the inner walls of the esophagus, and which are inflated to tamponade blood vessels along the esophageal wall. These devices are neither electrical adjuncts to defibrillation countershock, cardiac pacing or monitoring, nor mechanical adjuncts to CPR for treating cardiac arrest patients.
The need exists for an apparatus which enhances artificial circulation, and particularly enhances myocardial and cerebral perfusion, as well as simultaneously providing a more effective current pathway for defibrillation countershock without opening the chest by minimizing the distance between the electrodes and by positioning the electrodes so that there is virtually no tissues other than the myocardium interposed between the electrodes during the countershock.